Bring the cherries, sugar, lime zest and juice to the boil in a small pan. Reduce the heat, simmer for approx. 4 mins., pour into a measuring jug, allow to cool slightly then puree.
Pour the mixture into a stainless steel bowl, cover and freeze for approx. 5 hrs., scrape the mixture four times with a fork until just frozen. Divide the granita into glasses, serve immediately.